By the authority vested in me as President by the Constitution and the laws of the United States of America, including sections 121 and 12304 of title 10, United States Code, I hereby determine that it is necessary to augment the active Armed Forces of the United States for the effective conduct of Operation Atlantic…
The order comes on the heels of the president’s five-day trip to Europe, which wound through the U.K., Lithuania and Finland. The trip was meant to demonstrate the force of the international coalition against Russia’s invasion of Ukraine. And Biden appeared confident he had accomplished that mission, proclaiming that he and other NATO leaders showed the military alliance “more united than ever.”
The only news source I could find was Fox News: Biden authorizes military to use up to 3,000 reserve troops to augment US forces in Europe
The Washington Post is also reporting on it, but it is behind a paywall.
https://www.washingtonpost.com/national-security/2023/07/13/biden-pentagon-reservists-russia-ukraine/